Excepting the annoyingly talented, who can write and do their own cover, writers have to commission an artist.
This is a bit unusual, as writers have enormous freedom and do almost all their own work, from conception to scheduling, without anyone else necessarily being involved. (Beta readers might be asked to help, but again, this is entirely up to the writer and they can ignore any suggestions they dislike).
The cover is a massive deal, especially for those of us not quite as well-known as Tom Knox. It gives a clue as to the sort of book which is on sale and whether a browser might like it, and attracts those who might otherwise not be interested. One of my reviews on Amazon.com specifically stated that a combination of catchy title (Bane of Souls) and the cover got his attention.
Whilst there's a certain, and perhaps surprising, degree of collaboration between artist and writer ultimately the writer's got to trust their artist to do a job that is both good artistically and fits the vision of the world the writer has created. To be honest, it's a bit nervous waiting to see what the finished product will be like, but if you (well, the artist) get it right then it's not merely a relief, it's an active plus for the selling of the book.
The absolutely finished product of the Altmortis cover is not quite done but it's very close, and I'm delighted with it. Lee Yoong (http://leeyoong.deviantart.com/), who also did the excellent Bane of Souls cover, is responsible for it, and I hope that, when it's released, others like it as much as I do.
The map, however, is a different matter. I did consider asking her to do it, but decided against that because (correctly, as it happens) I imagined I'd end up changing the plot, moving bits and pieces around and if I've paid someone to do a job then it's unfair to ask them to do it again because I've changed stuff. In addition, the cover is what draws people's attention, whereas as a map is more of a nice to-have, and its prime purpose is functional (ie the cover matters a hell of a lot more). Lastly, the ability to do tiny bits in detail (individual mountains and trees, for example) means that the, er, artistically challenged, such as me, can do a reasonable job using the magic of computers.
The map is also very nearly done. I must confess, though, that Lee Yoong had a brilliant suggestion to improve it and is helping me a little bit in one regard. But it's mostly my work.